You need the Celestial Mirror. You can get the Cloudy Mirror by winning the chocobo race an Remiem Temple. Remiem Temple can be reached by heading right at the entrace of the Calm Lands while riding a Chocobo. When you see a feather on the ground, hit x and you should jump down to a lower level, which leads to the Temple. Once you have the Cloudy Mirror, you can turn it into the Celstial Mirror on the shimmering path in the Macalania Woods. You must first help the family that is there reunite. Once you have the Celestial Mirror, you will be given the option to use it at that location and you will recieve Auron's Celestial Weapon.